positional-defaults 2023.4.19

Set defaults for any positional-only parameter

positional_defaults

Python package to set defaults for any positional-only parameter

This tiny Python package contains a decorator @defaults which lets you specify default arguments for any positional-only parameter, no matter where it appears in the argument list.

Installation

pip install positional_defaults

Usage

Use the @defaults decorator to set default values for positional-only parameters (i.e. those before the positional-only indicator /):

from positional_defaults import defaults

@defaults(start=0)
def myrange(start, stop, /, step=1):
    ...

# now these are equivalent
myrange(4)
myrange(0, 4)
myrange(0, 4, 1)

This works on methods as well:

class A:
    @defaults(start=0)
    def myrange(self, start, stop, /, step=1):
        ...

Multiple defaults can be set, which are filled in the order in which they are specified:

@defaults(forename='Alice', greeting='Welcome', prefix='Mrs')
def greet(greeting, prefix, forename, surname, /, suffix='Esq'):
    ...

# these are now equivalent
greet('Smith')
greet('Alice', 'Smith')
greet('Welcome', 'Alice', 'Smith')
greet('Welcome', 'Mrs', 'Alice', 'Smith')
greet('Welcome', 'Mrs', 'Alice', 'Smith', 'Esq')

Signatures

Left-defaulted functions come with the correct signature:

>>> from inspect import signature
>>> signature(myrange)
<Signature (start=0, stop, /, step=1)>
>>> signature(greet)
<Signature (greeting='Welcome', prefix='Mrs', forename='Alice', surname, /, suffix='Esq')>

These show up correctly in the usual places such as help():

>>> help(myrange)

Help on function myrange:

myrange(start=0, stop, /, step=1)

>>> help(greet)

Help on function greet:

greet(greeting='Welcome', prefix='Mrs', forename='Alice', surname, /, suffix='Esq')

Performance

When the package is compiled as a native extension (which is what pip will deliver in the majority of cases), functions with positional defaults have comparable performance with respect to their undecorated functions.